The present invention relates to a brake booster for automotive vehicles comprising a low-pressure casing which is sealingly subdivided into a low-pressure chamber and a working chamber by an axially movable wall, a reinforcement tube extending axially through the low-pressure casing having its ends secured to the two transverse end walls of the low-pressure casing and sealed relative to the movable wall, a mechanically actuatable control valve to connect the working chamber to the low-pressure chamber or to atmosphere, the control valve being enclosed in an axially movable housing which via a push rod communicates with an actuating piston of a master cylinder fastened to the low-pressure casing on the transverse end wall adjacent to the low-pressure chamber, and communicates via a force-transmitting device with the movable wall, the force transmitting device extending through recesses of the reinforcement tube in the area of the working chamber as disclosed in copending U.S. application of H. Seip, Ser. No. 238,797, filed Feb. 27, 1981, assigned to the same assignee as the present application.
A light-weight construction in addition to a shortest possible overall length are aimed at in brake boosters for automotive vehicles. To enable the low-pressure casing to be constructed as light as possible, the force which develops during brake actuation and which is transmitted from the master cylinder onto the point where the brake booster is fastened to the automotive vehicle, for example, to the splashboard, will no longer be transmitted via the low-pressure casing, but via a central reinforcement tube according to more recent proposals.
To transmit the boosting force from the axially movable wall onto the control valve housing, a force-transmitting device is required which extends through recesses of the reinforcement tube.
These recesses may be arranged in the area of the low-pressure chamber of the brake booster as disclosed in U.S. copending application of J. Belart, Ser. No. 141,830, filed Apr. 21, 1980, now U.S. Pat. No. 4,347,779, issued Sept. 7, 1982 assigned to the same assignee as the present application. However, a difficulty is encountered in this construction since the recesses have to be placed outside the axial area which is overridden when the wall moves by the seal between the movable wall and the reinforcement tube, which can, for instance, be constructed as a rolling diaphragm. Otherwise, special covers or the like will have to be provided for these recesses to prevent the seal from being damaged. In this structure, the force-transmitting device is composed of a sleeve connected to the control valve housing and formed integrally therewith, from which sleeve extend radial ribs through the recesses of the reinforcement tube to establish a connection with a hub member of the movable wall.
An essentially less complicated construction of the force-transmitting device in the form of simple radial ribs extending directly from the movable wall to the control valve housing can be accomplished by an embodiment where the recesses of the reinforcement tube are disposed in the area of the working chamber as disclosed in U.S. copending application of J. Belart and F. Wienecke, Ser. No. 61,113, filed July 26, 1979, assigned to the same assignee as the present application. A slidable seal of the movable wall on the reinforcement tube is effected in the area adjoining the recesses on the side close to the low-pressure chamber. Since both the length of the recesses in the form of longitudinal slots and the length of the neighboring seal are at least equal to the power stroke of the movable wall, the length of the reinforcement tube is required to be greater than the double power stroke of the movable wall, which results in a comparatively large overall length of the brake booster.
To avoid the difficulties caused by the recesses in the reinforcement tube with regard to the sealing or to the overall length, it has also been previously suggested in a U.S. copending application of H. Seip, Ser. No. 238,797, filed Feb. 27, 1981, assigned to the same assignee as the present application to construct the force-transmitting device as bolts which extend from the movable wall through a seal in the transverse end wall of the low-pressure casing to a pressure plate fitted to the end of the control valve housing close to the pedal. In this arrangement, however, there is the difficulty of sealing the bolts in the housing end wall. Besides, the mounting space which is usually available for standard brake boosters in the engine compartment will be exceeded.